S.N.A.K.E.: A Dynamically Reconfigurable Artificial Sensate Skin ...

therefore defines its stability. Overall resistance is an issue if the amount of current flowing
through the gage needs to be controlled. Finally, The grid pattern is important because it
determines the axis of the strain to be measured; grid patterns can be either one of the typ-
ical ones (uniaxial, biaxial rosettes, multiple-axis rosettes, radial rosettes) or a customized
design. This is provided, however, just as an overview of the available design choices that
must be made; for a detailed explanation of these the reader can try Vishay’s tech note
TN-505-4[24].
For the more specific case of the current project, the strain gage design can be seen in
Figures 3-14 or C-7. An observant reader may notice that there are actually four sets of
traces, however, these are considered as two gages because the top and bottom sets of traces
are connected so they actually make one sensor; this is also the case for the left and right
sets. Their design characteristics are outlined in the following list:
Length: designed to cover as much area as possible to diminish dead zones.
Trace width: made as thin as possible; the manufacturer imposes a 5mil trace width
limit without imposing a price increase, so this width was chosen.
Overall resistance: determined by trace length and material properties. Although trace
length was controlled to make both gages in each node have the same resistance,
fabrication variations caused them to have a variation from node to node of up to
20Ωs. The total gage resistances are then in the rage of 168 − 188Ω.
Grid pattern: 90-degree T-rosette style with redundant traces (2 sets of traces per strain
axis) which gives a resolution of 2 sensors per node.
Strain gages, however, produce very reduced output voltage swings 17 because the changes
in resistance due to strain are small. For this reason, they need to be conditioned by a
very high-gain amplifier in order to be adequately sampled by the microcontroller. The way
this is usually accomplished, and the way it was actually done in the project, is by using
17 Tens of millivolts
63